1. Field of the Invention
The present invention relates to a positively chargeable toner which is used for developing an electrostatic latent image in the electrophotographic process or the electrostatic printing process.
2. Description of the Related Art
Toners for developing electrostatic latent images are generally prepared by dispersing a colorant such as a dye or a pigment and a charge-controlling agent for imparting a positive or negative chargeability in a binder resin such as a styrene resin, an epoxy resin or a polyester resin and finely pulverizing the composition to an average particle size of about 10 to about 15 .mu.m.
As the special preparation process, there are known a process in which a pigment or charge-controlling agent is stuck to the surfaces of particles composed of a binder resin by utilizing a mechanochemical reaction, and a process in which a pigment and a charge-controlling agent are dispersed in a monomer as the starting material of a binder resin and a spherical toner is prepared by a suspension polymerization reaction.
As the charge-controlling agent customarily used for a positively chargeable toner, there are known Nigrosine dyes, quaternary ammonium salts and metal salts of higher fatty acids.
As the process in which an imidazole is used for a toner for developing an electrostatic latent image, there are known a process in which 2-aminobenzoimidazole is used as the negative charge-controlling agent (see Japanese Patent Application Laid-Open Specification No. 217055/86 and Japanese Patent Application Laid-Open Specification No. 259265/86), a process in which vinylimidazole is used as the outer shell of a capsule toner (see Japanese Patent Application Laid-Open Specification No. 187350/84) and a process in which an epoxy resin is used as the binder resin of a toner and an imidazole is used as the curing agent (see Japanese Patent Application Laid-Open Specification No. 294461/86 and Japanese Patent Application Laid-Open Specification No. 242960/87).
Of known positive charge-controlling agents, a Nigrosine dye is poor in the durability of the charge-controlling effect and has a black color, and this charge-controlling agent is not suitable for a color toner other than a black toner.
Furthermore, a quaternary ammonium salt or a metal salt of a higher fatty acid has a white color or a light color, and therefore, this charge-controlling agent is advantageously used for a color toner. However, since this charge-controlling agent is poor in the charge-controlling effect, a large charge quantity cannot be obtained by addition of a small amount of the charge-controlling agent.